There are several techniques presently in use for conducting tube inspections. These techniques can be divided into two main groups: traversing and non-traversing. The traversing methods employ a probe which can inspect only the portion of the tube in its immediate vicinity. In order to inspect an entire tube, the probe is tethered to a cable by which the probe is pushed all the way down from one end of the tube to the other, and then pulled back. Traversing methods are slow, prone to wear and tear of the probe, and eventual failure. One example of a traversing inspection method is Eddy Current Testing, and related methods such as Remote Field Testing and Magnetic Flux Leakage testing. All these traversing methods are electromagnetic methods, having varying degrees of accuracy. Another example is the widely known IRIS (Internal Rotating Inspection System), which is based on ultrasound. IRIS is based on use of a probe that scans the tube wall in a spiral manner using an ultrasound beam propagating in water. It is much slower than the electromagnetic methods and requires cleaning the tube wall down to the metal, which is an expensive process.
Non-traversing methods are based on inserting a probe a relatively short distance into a tube under test, and then applying a physical method for inspecting the entire tube from this location. One such method is Acoustic Pulse Reflectometry (APR). In the APR method, an acoustic signal (which could be, for example, but not limited to a pulse or a pseudo noise signal, swept sine, etc) is propagated through the air inside the tube. Any changes in the cross section profile of the tube create reflections which propagate back down to the probe where they can be recorded and later analyzed. APR has good results at detecting anomalies in the interior surface or cross-sectional profile of a tube, such as blockages, through holes, and circumferential changes in cross section of a tube as a few non-limiting examples. APR has several advantages: APR is fast, it can accurately assess blockages, and it is very sensitive to through-holes, for example. A reader who wishes to learn more about APR systems is invited to read U.S. Pat. No. 7,677,103, or US pre-granted publication number US2011-0166808, or U.S. patent application Ser. No. 13/403,984.
A second non-traversing method, known widely as the Guided-Wave (GW) method, is based on propagating mechanical waves within the tube wall itself. These waves can be, for example but not limited to, a torsional or longitudinal wave, and the excitation signal can be for example, but not limited to, a pulse or a pseudo noise signal, swept sine, etc. The GW technique is sensitive mainly to the degree of material loss. Any changes in the tube wall properties or dimensions will create a reflection which can be recorded and analyzed. GW is fast and sensitive to flaws on both the outside and inside surfaces of the tube.
Usually GW non-traversing inspection systems are used for inspecting tubes in which resolution is less critical. In order to improve the resolution of the inspection system high frequency and wide bandwidth is needed. Typically GW non-traversing inspection systems have limited bandwidth.